The chances of the oil gallery plug, under the camshaft, coming loose is pretty remote. Possible, but not my first guess.

More likely is the possibility that the cam was not installed carefully & it has bumped the welch plug at the back of the last cam bearing position, dislodging it & allowing oil to leak out.

I had always replaced this welch plug with a brass one & I would install it after the cam was installed.

Yes, I know that the likelyhood of a steel welch plug rusting out when it has oil against one face & just air on the other is less than zero but I just replace everything that is regarded as a consumable once the engine is dismantled to the bare block stage.